Endurance athletes are often plagued with the question of whether or not they should add more volume...or whether they should add more intensity...in order to improve. It's tough to generalize, but here are my feelings.

Increasing the volume of training...adding more total time training...is often the simplest and most effective road to improvement. Trainer Andrew Read talks it about it here. Andrew points out...and I wholeheartedly agree...that adding easy aerobic work is the first place to go to when you want to reach new levels of fitness.

If you are a 4-hour marathoner already doing 40-50 miles of running per week, this may mean adding walking to your program...or some less taxing aerobic alternative. It's important to be aware if running can actually be "easy" for you. If you race 5K at 8:00 per mile...running only gets truly easy when it's 10:30-11:00 per mile...and it may be difficult to actually run at that pace. So walking...or a combination of running and walking might be better a choice when you are adding easy aerobic work.

At some point, intensity is required in order to improve. I can't tell you exactly when that is...but it's usually after you've built up a steady "base" of aerobic work over several years. If you've been running/walking at a comfortable pace for an average of 7 hours per week for several years...and your improvement seems to have stalled...and you are not training too fast during those 7 hours...which is always a distinct possibility...you might benefit by ratcheting up the effort level once or twice a week. This is where a good coach can help.I recently read an article by Steve Magness about world class marathoner Moses Mosop's training. You can read it here. In the article, Steve breaks down Moses' training into percentage of miles done at certain effort levels. The first thing I noticed was that over 52% of his 116 miles for the week were at what was termed "Regeneration" pace...very easy...at least 25% SLOWER than his MARATHON pace. Another 28% of his miles were at what was termed "base aerobic mileage"...12-25% SLOWER than MARATHON pace. Another 10% was at marathon pace to ~12% slower than marathon pace. That's 90% of the miles slower than his marathon pace....or to put it more accurately...90% of his training miles were at a pace he could hold for a little over 2 hours.Even though you are probably not a world class marathoner...I feel like you can apply similar ratios in your own training...once you have built up several years of a strong aerobic basis. Just ask yourself what pace you can hold for a little over 2 hours and go from there. For many...2 hour pace is close to half marathon pace.

Here are some additional points:

If you are improving just running easy, keep doing it until the improvement stops.

A little speed work goes a long way. Even when you add high intensity training, it should only constitute a small percentage of your training.

If you do more truly easy miles, you'll be able to handle more hard running. Keep things in proportion.

Finally...it's my observation that increasing intensity often leads to breakthroughs...short-term breakthroughs. Increased volume has more long-term results...but they are far more transformational...if that makes sense.Please let me know if you have questions.